Pass X-Registry-Auth when building an image
Pass X-Registry-Auth when building an image

* Initialize headers variable in BuildApiMixin.build() as a dict rather
  than as None.  This way the correct object gets passed to
  _set_auth_headers() even if no headers were set in build()

* Changing object from None to {} in BuildApiMixin._set_auth_headers()
  removed because it changes the object reference, so has no effect on
  calling code.

Signed-off-by: Justin Michalicek <36d65e5d3b710c49e5ab8f20c9dae1c2b17b49eb@gmail.com>

dimaspivak/docker-py,funkyfuture/docker-py,vpetersson/docker-py,dimaspivak/docker-py,minzhang28/docker-py,bfirsh/docker-py,bfirsh/docker-py,docker/docker-py,vpetersson/docker-py,youhong316/docker-py,auready/docker-py,docker/docker-py,youhong316/docker-py,vdemeester/docker-py,minzhang28/docker-py,kaiyou/docker-py,funkyfuture/docker-py,auready/docker-py,vdemeester/docker-py,kaiyou/docker-py